The increasing demand to improve fuel economy, eliminate emissions, and reduce noise levels has driven the automotive market to develop a variety of propulsion mechanisms. As an alternative to the internal combustion engine (ICE) the industry has developed a hybrid electric system powered by an electric traction motor(s) and an internal combustion engine. During varying driving conditions, hybrid electric vehicles (HEVs) will alternate between the separate power sources, depending on the most efficient manner of operation of each source.
An HEV may contain either a parallel drivetrain configuration, a series drivetrain configuration or a combination of series and parallel configurations. The configurations allow the ICE to perform relatively more efficiently than its conventional counterpart. In a parallel hybrid vehicle, the electric motor works in parallel with the ICE to combine the power and range advantages of the ICE with the efficiency and the electrical regeneration capability of an electric motor. The ICE drives the wheels through a transmission or transaxle. In a series hybrid vehicle, the ICE drives a generator to produce electricity for the electric motor, which drives the transmission or transaxle. This allows the electric motor to assume some of the power responsibilities of the ICE, thereby permitting the use of a smaller and more efficient engine.
Generally, a conventional automatic transmission, clutch-to clutch auto transmission and/or manual transmission is coupled to the internal combustion engine. The transmission is the mechanical component of a vehicle that transfers power (torque) from the engine to the wheels. The automatic transmission accomplishes this task by providing a number of forward gear ratios that automatically change as a function of vehicle speed and/or throttle position. The variety of gear ranges provided is made possible through the interaction of numerous mechanically, hydraulically and electronically controlled components inside the transmission.
The fuel economy benefits of certain HEV configurations are derived from the vehicle's ability to quickly and smoothly stop and restart the engine, as well as regenerative braking. A parallel hybrid vehicle will turn off the ICE during a normal engine idle period such as static or stopped condition. When the brake pedal is released, the engine will restart.
In order to achieve a smooth vehicle launch during the “engine-off” idle, adequate transmission oil line pressure must be maintained to prime the transmission and eliminate jerk upon a vehicle launch. This line pressure provides system cooling, torque transfer, and is the primary control medium of an automatic transmission. Normally, in a non-hybrid vehicle, the engine will spin the transmission oil pump, which will provide adequate line pressure. In many hybrid applications, a separate auxiliary electric pump is often added to maintain the line pressure when the engine is off. This auxiliary pump adds complexity and cost to the hybrid powertrain system.